The field concerned by the invention is that of thermostructural composite materials, and more particularly ceramic matrix composite (CMC) materials. These materials are characterized by their mechanical properties which make them suitable for building structural elements, and by their ability to maintain these mechanical properties at high temperature. Thermostructural composite materials are used in particular for making parts that are subjected to high thermomechanical stresses in aviation or space applications, e.g. parts of engines or fairing elements, or in friction applications, e.g. disk brakes for land vehicles or for aircraft.
CMC type thermostructural composite materials are constituted by fiber reinforcement densified with a matrix, the reinforcing fibers being of a refractory material such as carbon or a ceramic, and the matrix being a ceramic. Densifying the fiber reinforcement consists in filling the accessible pores of the matrix. It is performed by chemical vapor infiltration or by impregnation using a liquid precursor for the matrix and then transforming the precursor, generally by heat treatment. An intermediate coating or "interphase", in particular of pyrolytic carbon can be deposited on the fibers to optimize bonding between the matrix and the fibers, e.g. as described in document EP-A-0 172 082.
It is necessary to protect thermostructural materials against oxidation, particularly when they contain carbon, even when carbon is present only in an interphase between ceramic fibers and a ceramic matrix. The thermomechanical stresses to which such materials are subjected in use inevitably give rise to the matrix cracking. The cracks then provide access for oxygen in the ambient medium all the way to the core of the material.
A well-known method of protecting composite materials against oxidation consists in forming a coating having self-healing properties, which coating may be external or internal, i.e. it may be a coating anchored in the residual accessible pores. The term "self-healing" is used herein to designate properties whereby the material at its operating temperature passes to a viscous state that is sufficiently fluid to fill cracking of the matrix and thus block access to ambient oxygen. The self-healing coatings used are typically glasses or vitreous compounds, or else precursors therefor, i.e. substances capable of forming a glass by oxidizing at the operating temperature of the composite material (in situ glass formation).
Proposals have also been made in document FR-A-2 688 477 to form at least one continuous phase at the surface of the matrix or within the matrix, which phase is constituted by a ternary Si--B--C system. The relative proportions of silicon, boron, and carbon are selected so as to make it possible, by oxidation, to form a glass having the required viscosity characteristics for healing cracks at the intended operating temperatures, which temperatures may be as much as 1700.degree. C.
Undeniably, that protection technique considerably increases the lifetime of thermostructural materials in an oxidizing atmosphere. Nevertheless, it has been observed that protection is less effective at intermediate temperatures, i.e. about 450.degree. C. to 850.degree. C., than it is at higher temperatures.